The present invention relates to a device for scanning a target with a beam of charged particles, the device enabling the irradiating dose level received by the target to be modified in a predetermined manner.
A scanning beam is produced by subjecting a beam of charged particles to the influence of an electric field or magnetic field whose amplitude is a time-variable function, the variations generally being periodic in nature (alternating sinusoidal or sawtooth field for example). If the beam of particles in subjected to a magnetic field produced by a bi-polar electromagnet, scanning is obtained along a straight line whilst if the electromagnet is of the quadripolar kind, the beam can scan an area. However, it should be pointed out that the position occupied by the particle beam at the entrance to the quadripole device, in relation to the latter's axis, determines the distribution of the radiation dose level received by the target. In other words, any eccentricity in the beam at the entrance to the scanning quadripole device, introduces a modulation in the dose level from one edge of the target to the other, the target being disposed perpendicularly to the axis of the quadripole device and being centered in relation thereto.
It is therefore possible to modulate the radiation dose received by a target, in a predetermined manner, by arranging for the beam to be eccentric at the entrance to the scanning quadripole device.